Vacuum Interrupters and Embedded Poles for Medium Voltage (2)

Successful production and marketing of vacuum interrupters requires optimum chamber design, modern process-controlled production facilities and a product portfolio tailored to suit the market with its broad range of different requirements.

Vacuum interrupters are predominantly used for rated voltages up to 40.5 kV and short-circuit breaking currents up to 80 kA. Together with perfect function at the selected ratings, the compactness of these components is especially decisive, as they determine the overall size of the whole switching device.

A modern vacuum interrupter consists of two switching contacts in copper-chromium composite material (CuCr), one of which is fixed and the other movable. The contacts are located in a cylindrical casing which encloses an ultra-high vacuum (UHV) of <10–7 hPa. The vacuum interrupter chamber consists of a ceramic insulator, a metal bellows on the moving contact side and a concentric shield to prevent the condensation of metal vapour on the inside of the ceramic material during an opening process.

As soon as the contacts in a vacuum interrupter are separated by the breaker's operating mechanism, a switching arc (metal vapour arc) begins to form, and is interrupted at the next current zero. A special contact geometry ensures that the arc is moved in the gap between the contacts and the energy the arc contains transmitted to the contacts and the shield.

The internal structure adjusted to suit the prevailing switching capacity and the uses of special materials are the preconditions for reliable and robust function of the vacuum interrupter. With the use of a vacuum as the switching medium, relatively small contact gaps of around only 8 mm to 14 mm are sufficient, with a corresponding beneficial effect on the cost-effectiveness of the operating mechanism.

Together with a large number of potential breaking operations on the full rated short-circuit breaking current, other parameters such as mechanical service life also place demands on the quality of vacuum interrupter production and testing. In order reliably to achieve a service life of 30 years of more ("Sealed for Life"), extremely stringent quality requirements have to be applied to every individual soldered joint, some of which are between different materials, to maintain the ultra-high vacuum.

As a result, production in clean room conditions and the necessary routine testing of the dielectric properties and internal pressure of vacuum interrupters are characteristic features of interrupters of the highest quality. These demands can only be met by process-monitored series production, supported by a strictly implemented Quality and Environmental Management System to DIN EN ISO 9001:2000-12 [6] and DIN EN ISO 14001:2005-06 [7].

ABB supplies a broad range of latest generation vacuum interrupters whose compact design and versatility in use satisfy the requirements of the market in full. They can be combined with other platform technologies such as embedding, and are thus suitable for service in a broader spectrum of applications.

Series VG for vacuum circuit-breakers covers applications with rated voltages Ur in the range of 12 kV to 40.5 kV and is suitable for rated currents Ir of up to 4 000 A and rated short-circuit breaking currents Isc of up to 63 kA. Even the 63 kA vacuum interrupter variant is extremely compact in design, with a diameter of only 142 mm and a length of 224mm. All the vacuum interrupters of this series fulfill the highest mechanical, electrical and capacitive switching requirements M2, E2 and C2 to IEC 62271-100:2006-10 [8] and DIN EN 62271-100 (VDE 0671-100):2004-04.

The vacuum interrupters of the VS series are designed for contactors and switch-disconnectors in the rated voltage range from 3.6 kV to 24 kV and for up to 1000000 mechanical switching operations (contactor applications) or 15000 mechanical switching operations (switch-disconnector applications). The vacuum interrupters comply with requirements AC-1, AC-2, AC-3 and AC-4 to IEC 60470:2000-05 [9] and DIN EN 60470 (VDE 0670-501): 2002-01.

Apart from the requirements of medium voltage networks, operated differently throughout the world, the vacuum interrupters from the portfolio outlined above also fulfill the requirements for railways and further special applications.